The present invention relates, in general, to a broad class of transducers which are herein referred to as "electro-mechanical" transducers. It is intended that this class of transducers includes microphones, speakers, relays, linear or rotational motors, and even servo-motors or other transducers wherein the mechanical response characteristics of the transducer include some mass or moment of inertia, some elasticity, such as a spring suspension system, and some loss or dissipation.
It is known that transducers of this general type have an inherent dependency upon, or sensitivity to, the frequency of the signal which drives the transducer. The present invention is particularly directed to compensate for, and therefore substantially eliminate the effects of this inherent frequency dependency of the mechanical response of a transducer.
In a general sense the present invention relates to a class of devices which operate on the Bli principle--namely a force is produced on a current-carrying conductor in a magnetic field. The meaningful output may be velocity as in the case of a moving coil type of speaker or it may be displacement as with the recording head for a chart recorder.
In a speaker of the type mentioned, there is mass (the mass of the cone and transducer coil), and elasticity or spring parameter (in the suspension system for the cone and coil), and some loss (including the dissipation in the cone and the work performed in vibrating the air to generate the acoustic radiation).
These three parameters of mass, spring parameter, and loss are referred to herein as the "mechanical response parameters" of the transducer. As mentioned, they inherently produce a frequency dependency of the mechanical response to an electrical signal.
In addition to the mechanical response parameters indicated above, a transducer of the type with which the present invention is concerned, particularly audio speakers, has an ohmic loss and self-inductance which may alter the response characteristic of the transducer.
The present invention can best be understood from the detailed analysis of the problem and specific solution presented below; however, in brief, the present invention provides a current-controlled power amplifier (that is, one which has a very high output impedance in comparison to the load so that its output current is substantially independent of load impedance) for driving the speaker. In other words, a "current-controlled amplifer" is one in which the output current of the amplifier is controlled by the input signal (either voltage or current) and it is independent of load impedance. In this manner, the impedance of the transducer coil caused by its ohmic resistance and self-inductance have no distorting frequency effect because the current-controlled power amplifier forces the desired current into any electrical load having an impedance value from a short circuit (zero impedance) to the highest impedance within the power rating of the amplifier.
The use of a current-controlled power amplifier to drive the speaker, therefore, overcomes the frequency-dependency of the electrical parameters of the speaker coil, but it does not compensate for the frequency dependency of the mechanical response parameters of the speaker.
To compensate for the frequency dependency of the mechanical response parameters of the speaker, the input signal of the current-controlled amplifier (or simply "current amplifier") is modified to have a frequency characteristic which is the complement of the corresponding frequency characteristic of the mechanical response parameters of the transducer. In this context, term "complement" is intended to mean that the input signal is large when the mechanical response is small and vice versa. Detailed analysis presented below will more precisely define the meaning.
In the illustrated embodiments, this modification of the current drive signal is accomplished through the use of a scaled electrical network which is a model of the mechanical response parameters of the transducer. That is to say, the input signal to the current power amplifier is derived by applying the input voltage signal, after amplification, to an electrical network, the elements of which are selected and designed to be the electrical counterparts of the equivalent circuit of the actual mechanical parameters of the speaker. This is sometimes referred to as the "modeled electrical network" or "compensation network"; and it will be explained in greater detail within. In summary, then, the input electrical signal acts upon the modeled electrical network so as to modify that input signal in such a manner that the resulting signal has a frequency characteristic which is the complement of the corresponding frequency characteristic of the mechanical response parameters of the speaker. This resulting signal is then used to drive a current amplifier which, in turn, drives the speaker.
The result is an overall response for the system which is substantially independent of frequency.
The present invention finds particular utility in the field of high-fidelity sound reproduction where heretofore the principal element of such a system limiting fidelity of the reproduction has been the speaker. In order to overcome this problem and to achieve better fidelity, the effort has been to design better speakers because the frequency response of the electrical portion of the system has not been matched by the frequency response characteristic of the speakers. The present invention, then, provides an economical and convenient method of modifying the overall system response for any speaker, even an inexpensive one, to achieve system performance with a much improved frequency response, and therefore higher fidelity. As mentioned, this may be accomplished even with inexpensive speakers.
Further, in order to achieve improved results, the modeled electrical network need not match perfectly with the frequency characteristic of the mechanical response parameters of the speaker at all frequencies of interest. An improved response will be obtained, as will be discussed, if the matching occurs at only a finite number of frequencies such as at resonance, at a frequency lower than resonance, and at one or more frequencies higher in the audio spectrum.
Another advantage of the present invention is that it permits simulation of any of the mechanical response parameters which may be non-linear or itself have a frequency dependency. Further, such simulation may be accomplished in the exact circumstances in which the speaker is intended to be used. For example, I have found that slight variation may occur in the mechanical response characteristics of the speaker when it is enclosed or mounted in a cabinet.
In still another aspect of the invention, there is disclosed a method for determining the exact values of the elements in the modeled electrical network for a given speaker.
The audio frequency response to a room in which the system may be operated, may in itself be imperfect. The room response may be modeled by the methods of this invention and compensation may be included as part of the total electrical model to provide improved audio response of the total system composed of the speaker system operating in the room.
Other features and advantages of the present invention will be apparent from the following detailed description accompanied by the attached drawing.